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Unreal source in lense

  1. Apr 10, 2013 #1
    I was having trouble while thinking about the unreal source for refreaction through a lense... In lense light is refracted twice...the real image for the first refraction works as an unreal source for the second refraction... I understand how it is... But fail to feel it...
    I mean how would I see the unreal source as? I mean to say what is the diiference between real and unreal source... from a real source light really comes out,it seems to come from a unreal surce bu actually it doesn't... How would I know the difference between this two...

    I know there's no difference to us for real and unreal image...

    I think I am having a little discomfort to regard the source as unreal...
  2. jcsd
  3. Apr 10, 2013 #2

    Simon Bridge

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    "unreal" is not a term in optics that I am familiar with
    An image is "real" if you can put your finger on it, resolve it on a screen etc. and "imaginary" otherwise.

    I think the principle you are talking about is where the image formed by the first surface (or a fat lens say) is used as an object for the second surface.

    What you are doing is clearer when you use ray-tracing with pencil and paper.
    The three principle rays you use are not the only rays - they are just three convenient ones ... but they are only convenient for the first surface. To work out where the actual image will end up, you need to find the three rays from the object which, when refracted by the first surface, will be principle rays for the second surface.

    If you do the analysis and then trace - in bold - the actual path of the light through the lens, you should see how working out the location of the image from the first surface helps you do this.
  4. Apr 11, 2013 #3
    I have drawn the picture... When using spectacles,I understand how the imaginary source works... Cause the light comes to me such a way that I think its coming from a nearer position... So my eyes can see objects coming from infinitely distant places(when my eyes have defect)...

    But my problem is with lens's case... Here after the first refraction light would go a 'certain point'... But for the 2nd refraction it goes to another point... Here how will it seem that the light is coming from that 'certain point'... I solved the equations... proved the laws... But having problems thinking about
    "how will it seem that the light is coming from that 'certain point'"
  5. Apr 11, 2013 #4
    I think the 2types of image are 'REAL' and 'VIRTUAL'
    Real images are formed by converging light rays and can be projected onto a screen
    Virtual images are formed by diverging light rays and cannot be projected onto a screen
  6. Apr 11, 2013 #5
    The only way to distinguish between the two is that if you actually physically go to the point from which the light rays appear to be coming, you will find nothing there if it is an unreal source. Whereas if it is a real source you will actually find the bulb or lamp there. Thats how mirages are formed btw - with the atmosphere bending light rays to make the source appear closer than it really is, but when you get to the spot where the source appears to be - you find nothing!
    From a distance and based on purely optical properties there would not be any way of distinguishing the two.
    You only have to try and grab an underwater object to see how deceptive this can be.
  7. Apr 11, 2013 #6
    This is incorrect. Some images are at infinity...you cannot go there.
    The images are called REAL or VIRTUAL
    I have grabbed many underwater objects
    Spear fisherman have caught many fish
    Last edited: Apr 11, 2013
  8. Apr 11, 2013 #7

    Simon Bridge

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    Light will appear to be scattered from the image position.
    So you can see how the light gets through the optical system ... you traced the actual path of the light ray. Then to see why it appears to be coming from "that certain point", all you need to do is trace at least two exiting rays back in a straight line.

    Your visual system basically does ray-tracing from the light that hits your eyes to work out where stuff is.
  9. Apr 12, 2013 #8
    "Your visual system basically does ray-tracing
    from the light that hits your eyes to work out
    where stuff is."

    That is the main thing... I found it out myself by thinking how images are formed... And I fully agree with physwizard ...

    I did what u told me before... But I fail to grasp how I can observe the source to be behind me... For say, u are sitting in front of a lense... I light a torch from the other side... Light gets refracted and reaches u... For the first refraction I am the source... And light is going towards a certain point... Now if u were at that point where would u see me? At my place or a little higher... (I don't think our eye can trace lens rays back so I think a little higher? Am I wrong? But when u look at me u will be looking through the lense... So?)
    However, after the second refraction light reaches u,now how will u say that "certain point" is ur source... How will ur brain apprehend the source to be behind u?

    Really confused... :(
  10. Apr 13, 2013 #9

    Simon Bridge

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    You cannot.
    Light rays diverge from the image. Diverging light has to hit your eye for you to be able to see an image.
  11. Apr 13, 2013 #10
    U r right... I am saying then how will u say the certain point is ur source?

    And what's ur answer abt my other ques...
  12. Apr 13, 2013 #11

    Simon Bridge

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    The source of light is the glowy thing.
    The object is where the physical light rays diverge from.

    If you do not know what is between you and the source/object, then there is no way to tell where it is. just by looking at it. All you can see is the image.
    Usually, however, we have some ideas about this that let is make refined guesses. If we've set up the optical equipment, then it is simple. That is why you are being taught all this stuff about optics - and, for tha matter, physics - so you'll have the tools to figure out real world situations which are less cut-and-dry.
    Last edited: Apr 13, 2013
  13. Apr 16, 2013 #12
    can u please elaborately describe the true and seemingly position of source while refracting through a lense... I mean where the source really is and where it seems to be...
  14. Apr 17, 2013 #13

    Simon Bridge

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    What do you mean by "source"?
    Light rays arriving at the eye appear to the observer to diverge from the image.

    You've seen ray diagrams done before surely?
    How would you normally locate the image?

    Here's one for two thin lenses:


    ... the lenses are the vertical lines.
    To an observer to the right of the I2 position, the three principle rays drawn (and all the others) will appear to diverge from the head of the arrow drawn at the I2 position.

    Image comes from this "optics tools" website
  15. Apr 17, 2013 #14
    I thought about it and understood it... Actually I was confused about unreal source for the 2nd refraction and that led to other ones...

    We know that a certain thing reflects light and we see it... So when this phenomenon is happening from any point we think the thing is there... For lense after the first refraction light was converging to a certain point... That would be the image,seeming position of source... But after 2nd refraction the image is set nearer... There the certain point is working as a source... Hence unreal source term is used as for real there's none there...

    And another question... Suppose the observer is nearer the image point... Now where will he see the source?
  16. Apr 17, 2013 #15
    I mean where he will think the source to be...
  17. Apr 17, 2013 #16

    Simon Bridge

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    You keep saying "source" - I don't know what you mean?
    I cannot help you if you won't answer questions.
  18. Apr 17, 2013 #17
    My last question was---
    "the observer is nearer the image point... Now where will he see the source?"

    Here by source I mean the true source of light... The image is there where light meet and I think the source to be there... In this case, the point where the image is created is behind me... So where will I see the source to be?
    I think I will see the source(from where light is created) to be just a little up from its actual position...
  19. Apr 17, 2013 #18
    Okay now that I read my question,it seems really vogue... I apologise... I should choose my words more wisely... My ques should be---

    "the observer is nearer to the lense than the image point(point where light rays were converging)... Now
    where will he think the light rays are coming from(diverging from)?"
  20. Apr 17, 2013 #19

    Simon Bridge

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    Oh OK :
    a magnifying glass is a converging lens - it produces real inverted images of distant objects.
    But it you hold it right up to your eye to look at a distant object, the image position is clearly behind your head someplace isn't it? So what do you see? (If you havn't done it before, go find a magnifying glass and do it.)

    Is that the sort of thing you are thinking of?
  21. Apr 18, 2013 #20
    Yes that's what I am asking for...

    will it get blurry?
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